In the reference book “Otto-Motor—Management/Bosch”, Vieweg Publishing House, 1st edition, 1998, pages 22-23, a broadband lambda sensor is described, having a sensor chamber which is connected to a gas compartment via a diffusion barrier. Situated in the sensor chamber is an inner pump electrode which, together with an outer pump electrode and an electrolyte that is situated between the pump electrodes and conducts oxygen ions, forms a pump cell. Using the pump cell, oxygen ions of the gas can be pumped through the electrolyte out of, or into, the sensor chamber.
In addition to the pump cell, a measuring cell is provided that is disposed between the inner pump electrode and a reference-gas electrode, an electrolyte conducting oxygen ions likewise being situated between the inner pump electrode and the reference-gas electrode. The measuring cell corresponds to a Nernst cell, in which the potential difference forming in the thermodynamic equilibrium between the inner pump electrode and the air reference electrode is proportional to the logarithm of the relationship of the partial pressure of the gas to be analyzed in the sensor chamber and the partial pressure of the gas to be analyzed in the air reference.
A circuit configuration realized in analog circuitry has the task of influencing the oxygen partial pressure in the sensor chamber in such a way that the Nernst potential remains constant at a predefined value. For this purpose, the circuit configuration alters an electrical pump current which acts upon the outer pump electrode. The polarity and the amount of the pump current depend upon whether and by what amount the predefined Nernst potential is exceeded or not attained. The pump current obtained appears at a load resistor as voltage, which is a measure for the concentration of the gas to be analyzed.
The Published German Patent Application DE 36 25 071 describes a method for operating a sensor having variable ionic conductivity, as well as a device for implementing the method, in which the sensor, in cyclically proceeding operations, is acted upon in pulse-like fashion by a current supplied from a constant current source. The resulting voltage at the sensor is a measure for the relative atmospheric humidity to which the sensor is exposed. The pulse-like feeding of the current prevents polarization effects at the electrodes of the sensor. The use of a constant current, which can have an alternating polarity, permits a simple evaluation of the sensor voltage resulting at the sensor element.
An object of the present invention is to provide a device for operating a gas sensor, which contains a circuit configuration that is easy to implement and permits a precise, low-drift and low-offset measurement